Various diene-based elastomers may be prepared, for example, by blending the elastomer(s) with rubber reinforcing filler such as rubber reinforcing carbon black and silica reinforcement, particularly precipitated silica, together with a coupling agent comprised of a bis(3-triethoxysilylpropyl) polysulfide to aid in coupling the silica to the elastomer and enhancing its rubber reinforcing effect. Preparation of such rubber compositions are well known to those having skill in such art.
In an alternative embodiment, the precipitated silica may be pre-treated with a silica coupling agent comprised of, for example, an alkoxyorganomercaptosilane or combination of alkoxysilane and alkoxyorganomercaptosilane prior to blending the pre-treated silica with the rubber instead of reacting the precipitated silica with the silica coupling agent in situ within the rubber. For example, see U.S. Pat. No. 7,214,731.
For various pre-treated precipitated silicas see, for example, U.S. Pat. Nos. 4,704,414, 6,123,762 and 6,573,324.
However, it has been observed that a rubber composition loaded with (having a relatively high concentration of) such pre-treated precipitated silica can provide a cured rubber composition with insufficiently desirable stiffness and, may provide an uncured rubber composition having an undesirable and readily observable largely grainy physical appearance which is considered herein to be undesirable in a sense of an observed difficulty in banding the somewhat granular condition, often occurring in individual clumps, of the rubber composition on a cylindrical mill as well as calendering and extrusion.
For this invention, it has been discovered that an addition of, and therefore an additional treatment of the already pre-treated silica in situ within the rubber composition with a combination or bis(3-triethoxysilylpropyl) polysulfides which has been observed to significantly increase stiffness of the cured rubber composition and also to substantially reduce, and tend to eliminate, the aforesaid undesirable grainy physical condition of the uncured rubber composition and to thereby render a more cohesive rubber composition.
In the description of this invention, the terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The terms “cured” and “vulcanized” may be used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of an ingredient per 100 parts by weight of rubber in a rubber composition.
Such terms are well known to those having skill in such art.